1. Field of the Invention
The present invention relates to a signal reproduction apparatus of an optical disk player, for example a compact disk (CD) player, and more particularly, to an optical signal conversion apparatus in which peaking detected from an RF signal read from a disk is compensated in order to reproduce a stable output signal.
2. Description of the Related Art
In general, a CD player includes a preamplifier, a data strobe portion, a digital signal processor, an analog-to-digital (A/D) converter, and an analog circuit portion. The preamplifier amplifies a signal sensed by a photodiode, removes noise and distortion from the signal using a waveform equalizer, and then transmits the signal to the digital signal processor following waveform-shaping. The preamplifier commonly includes an adder, an automatic gain controller (hereinafter "AGC"), a waveform equalizer and a data slicer circuit.
The data slicer circuit shapes the signal waveform equalized by the waveform equalizer and converts the signal into a rectangular pulse. Since a slice level should be set at the center of an eye pattern, if the slice level is set at a level beyond the center of the eye pattern, an error is generated in the pulse width of the positive and the negative and accordingly a data error is generated. This asymmetry phenomenon commonly occurs when the degree of asymmetry in the CD data pits is greater than 20%.
The data slicer circuit commonly includes a correction circuit for correcting the asymmetry phenomenon. The eye pattern may have a defect due to a pin-hole generated during the disk manufacturing process. When the eye pattern is applied as an input to an eight-to-fourteen modulation (EFM) circuit and then passed through a passive high pass filter (hereinafter "HPF"), a peaking phenomenon of the eye pattern occurs wherein the eye pattern is not integrated instantaneously in the defect interval. Thus, in EFM slicing, since the EFM input does not trace DC offset instantaneously, it is sliced at an incorrect DC level thereof. The incorrect DC level causes troubles in error correction (hereinafter "ECC").
The EFM signal conversion apparatus used for the conventional optical disk reproduction system including a high frequency slicer, or an RF slicer, converts an RF signal into a digital EFM signal. A typical EFM signal conversion apparatus slices an RF signal received by a photodiode.
FIG. 11 is a circuit diagram showing a conventional EFM signal conversion apparatus. The EFM signal conversion apparatus includes a capacitor C, a comparator 110, operational amplifiers 112 and 114, resistors R1, R2, R3, and R4, resistors R5 and R6 and capacitors C1 and C2 constituting a two-stage low pass filter (LPF) 116, and invertors I1 and I2 constituting a MOS buffer 118.
In the operation of the EFM signal conversion apparatus having an eye pattern of a CD player as a non-inverting input of the comparator 110, a DC offset is extracted by passing the output of the comparator 110 through the two-stage LPF 116. The extracted DC offset is fed back as an inverting input of the comparator 110. The comparator 110 corresponds to the above-described data slicer circuit. As the eye pattern is applied as the input of the comparator 110, the eye pattern passes through an HPF differentiator.
The capacitor C shown in FIG. 11 as receiving an RFO signal output from the photodiode removes a DC component from the RFO signal and outputs the RFI signal removed of the DC component to the non-inverting input terminal of the comparator 110. Here, the comparator 110 compares the RFI signal input to the non-inverting input terminal thereof with the signal output from the operational amplifier 112 and outputs the comparison result as a digital EFM signal via an output terminal OUT. After being buffered in the MOS buffer 118, the digital EFM signal passes through the LPF 116, where DC offset is detected, and an asymmetry buffer and an asymmetry amplifier, and finally is fed back to the comparator 110. Here, the asymmetry buffer corresponds to the operational amplifier 114 of an emitter-follower type and the asymmetry amplifier consists of the resistors R1, R2, R3, and R4 and the operational amplifier 112.
In the conventional EFM signal conversion apparatus, since the asymmetry phenomenon occurring due to manufacturing deviation cannot be removed with only an AC coupling, it is used when the probability of generation of 1 or 0 according to the digital EFM signal is 50%. However, when a defect occurs such as a scratch or pin-hole, this technique is time consuming, as long as the external time constant, for example approximately 5 ms, in order that the slice reference level for the eye pattern tracks an accurate DC level for the eye pattern following the defect interval.
Accordingly, during the period of an input signal where there is no eye pattern or wherein the eye pattern is defective, when the abnormal signal portion is differentiated, the DC level of the eye pattern may instantaneously peak. Since it normally takes a long time to extract a DC offset, the eye pattern cannot be sliced during this brief period. Namely, in the conventional EFM signal conversion apparatus, when an optical disk contains a scratch or pin-hole defect, it is a problem that it takes a time as long as the external time constant for the slice reference level output from the operational amplifier 112 to trace an accurate DC level of an RF signal.
Further, since the asymmetry of the RF signal is not appropriately compensated during the period where defects exist, an error correction portion connected to the rear end of the EFM signal conversion apparatus is prevented from accurately correcting errors.